There are many compounds that have suitable fire-extinguishing properties but which cannot be used in fire extinguishers for use on Class A fires (i.e. paper and wood) because their exit velocities from the extinguisher nozzle are too high. These compounds have in common the fact that they have very high vapor pressures at room temperature (i.e. 500-800 psi). They are stored in a fire extinguisher as a liquid, and when directed at a fire the liquid comes out of the nozzle as a gas at a very high velocity.
Liquid carbon dioxide is one of these compounds. Carbon dioxide is well known for putting out fires, but the exit velocity from a prior art nozzle is so high that burning paper and small pieces of wood are blown about and scattered rather than being extinguished. This propagates, rather than extinguishes, the fire. Even when some of the CO.sub.2 is converted to "snow" the exit velocity from prior art nozzles is still too high to be used on paper or wood fires.
Another compound with the same undesirable exit velocity is HFC-23. This is quite effective in putting out fires, but its high exit velocity also prevents it from being used on paper or wood fires.
A further problem with using prior art nozzles with these high vapor pressure compounds is that when they are used in a closed compartment--an aircraft cargo compartment, for example--the pressure rise in a total flood system is so sudden and so great that it can rupture the compartment because of the large volume of gas needed. Thus the cargo compartment extinguishing systems that are used on aircraft must contain Halon, which, while it is very effective and does not have the disastrous pressure rise, is damaging to the ozone layer and hence is being phased out.
Some CO.sub.2 extinguisher nozzles produce a small amount of "snow", which is desirable since it slows down the high exit velocity and sudden pressure rise. However, these nozzles do not produce enough snow to allow their use either on Class A fires or in aircraft cargo compartments.
The prior art shows means for producing CO.sub.2 snow or "dry ice" which is used for preserving perishables wherein the liquid CO.sub.2 undergoes an expansion and then passes over the inlet for the liquid CO.sub.2 (see, for example, U.S. Pat. No. 4,145,894). However, this does not convert enough liquid CO.sub.2 to snow to reduce the exit velocity if it were used as a fire extinguisher.